The invention relates to the manufacture of radiation detectors comprising a set of individual xe2x80x9cmicrodetectorsxe2x80x9d arranged in matrices or line arrays. The invention is particularly advantageous in the case where these microdetectors are microbolometers.
A bolometer is a device designed to transform the radiation to which it is subjected, typically in the infrared range, into thermal energy. The resulting heating of the bolometer gives rise to the variation in an electrical property, for example the electrical resistance of a conductor connected to a circuit exterior to the bolometer. In the case of a detector comprising an array of microbolometers, said electric circuit, known as a xe2x80x9creadingxe2x80x9d circuit, manages the array addressing functions and the reading stimuli sent to each microbolometer, and converts the resulting signals into a format exploitable for imaging (for example in the form of a video signal). To obtain the best possible performance, the microbolometers are made to operate under relatively low gas pressure (or under moderate pressure of a gas having low thermal conductivity), in order for the thermal dissipation due to this gas to be negligible in relation to the inherent thermal conductance of the bolometers.
Typical methods of manufacture of detectors of this type comprise initial steps carried out directly on the surface of an electronic circuit, in a so-called monolithic manner (i.e. in a continuous sequence of operations on the same substrate, usually of silicon) or hybrid manner (with transfer onto a substrate of prefabricated elements). These steps involve common techniques of the microelectronics industry, in particular techniques of collective production, typically concerning several tens to several hundreds of detectors deposited onto the same substrate (wafer level). During these steps the components that are actually bolometric (optical absorption, and resistance variable with temperature) are mounted on the surface of a layer that is xe2x80x9csacrificialxe2x80x9d in the sense that this layer (usually made of polyimide, of polycrystalline silicon, or of metal such as copper or aluminum) is eliminated at the end of the process (by combustion in an oxygen plasma, for example), so as to leave the structures of the bolometer suspended above the substrate.
Further to these initial steps, an automated quality test and sorting operation are performed, then the assembly is cut up into individual detectors. The process of manufacture ends with a so-called xe2x80x9cunitaryxe2x80x9d set of operations, that is to say carried out on each detector individually.
These unitary operations typically comprise the following steps:
The first step is the xe2x80x9cfreeingxe2x80x9d of the microbolometers, which consists in eliminating at least a part of the sacrificial layers. This operation leads to the structures being extremely vulnerable mechanically. Moreover, the slightest contamination by dust (so-called xe2x80x9cparticulatexe2x80x9d contamination) of size greater than a few micrometers also deteriorates the detector locally, since cleaning cannot be envisaged, in any manner, due to the risk of complete destruction: more particularly, the freed microbolometers can withstand neither blowing, nor wetting nor contact.
Next, each detector is glued or soldered to a ceramic die carrier, this die carrier being itself usually glued or soldered onto a system of thermal regulation (Peltier element), and the detector is connected by wire bonding. This assembly is next mounted into a casing comprising at least two parts: a metallic or ceramic die carrier, and a cover comprising a window transparent to infrared radiation. Before sealing of said cover, the electric inputs and outputs of the casing are connected to the metallic tracks of the ceramic, again by wire bonding. The assembly and the number of operations may be optimized, but, as may be seen, this remains very complex overall.
Finally, for the systems of highest performance, before sealing the cover a xe2x80x9cgetterxe2x80x9d is placed within the casing. A getter is constituted by a material known for its capacity to improve the quality of the vacuum (for example iron, titanium, vanadium, cobalt, aluminum, zirconium or manganese, or an alloy of these metals).
All these unitary operations, associated with the supply cost of the elements and components making up the casing, result in an additional manufacturing cost which is considerably higher than the cost of collective manufacture of the detectors. This end of manufacturing cost (after the collective operations) is particularly high in the case of microbolometer imaging systems (it commonly attains 60 to 70% of the cost of the final component), since the level of residual pressure required for optimal operation of the detector (typically of the order of 10xe2x88x922 mbar) requires a high quality of vacuum sealing. This justifies the implementation of unitary techniques developed on specific casings (use of vents or evacuation pipes), and a relatively long cycle (several hours) of degasification and activation of the getter. Furthermore, the operations of testing and sorting of the products in course and/or at the end of manufacture are themselves unitary, and very difficult to automate.
It may thus be seen, in relation to the manufacture of detectors comprising arrays of microbolometers, that conventional techniques suffer from a low rate of production, and a high overall cost.
The object of U.S. Pat. No. 5,895,233 is specifically to improve the state of the art in this field. This document provides a collective technique for the production of covers on substrates of conventional dimensions which will be referred to as xe2x80x9cwindow substratesxe2x80x9d, and a technique, also collective, of sealing each window substrate onto a xe2x80x9cdetector substratexe2x80x9d, the two substrates being held at a certain distance from each other by means of solder beads so as to ensure the mechanical protection of the detector under vacuum.
This technique has several drawbacks. First of all, the soldering of the cover over the detector substrate is difficult to manage. Furthermore, the creation of the vacuum requires the use of getters of large size if an ordinary soldering apparatus is used, or otherwise a high technology vacuum soldering apparatus. Furthermore, this technique requires the use of relatively thick materials (several hundreds of microns) to produce the xe2x80x9cwindow substratexe2x80x9d, which limits the optical transparency, and therefore the performance of the detector. Another drawback is that the solder beads occupy a large surface area of the detector substrate, which means that only detectors of large size can be economically worthwhile; but in the case of large detectors, flexing of the two parts of the casing is observed under the effect of the external atmospheric pressure, which gives rise to geometrical aberration, and can even lead the window to come into contact with the microbolometers, in which case the detector is destroyed; to alleviate this problem, the document proposes to arrange a pillar within the casing, but this means that another part of the detector is made blind.
To solve these problems, the invention provides a method of manufacture of radiation detectors, in which the detectors each comprise an assembly of microdetectors, for example microbolometers, under a window that is transparent to said radiation, said method being remarkable in that said detectors are manufactured collectively on a substrate, and in that it comprises notably the following steps:
the construction of several layers, of which, for each of said detectors, at least one layer is transparent to said radiation and serves as a window, and
the partial elimination of said layers principally under said transparent layer, such that said microdetectors are placed, for each of said detectors, in one or more cavities, which are then placed under vacuum or under low pressure.
Thus, the detectors according to the invention comprise, due to their actual construction, the elements adapted to keeping the microdetectors under vacuum. Advantageously, this construction according to the invention may use xe2x80x9cmicroelectronicsxe2x80x9d techniques, that is to say all the techniques of micro-manufacturing such as the deposit and etching of layers.
According to particular features, said steps of construction and partial elimination of layers leads to the formation of a peripheral partition surrounding each of said detectors.
Thus, whatever dimensions have been chosen for the detector, there will be no loss of lateral space at the periphery to obtain a finished and protected component. The method according to the invention is thus equally attractive from an economic point of view for small detectors as for detectors of large dimension.
According to particular features, said step of partial elimination of layers is carried out through one or more openings formed in the envelope of each of said detectors. Preferably, said openings will be formed in said transparent layer, in order to facilitate the provision of these openings.
By virtue of these arrangements, the cross-construction of said cavities containing the microdetectors is particularly simple, and only requires conventional microelectronics techniques.
Preferably, said step consisting of creating a vacuum or a low pressure in the cavities will be carried out through said openings. Moreover, during said steps of construction and partial elimination of layers, the formation of a surface will be provided within the cavities, opposite each of said openings, serving as a support for a material capable of sealing said openings in order to seal said cavities.
By virtue of these provisions, the xe2x80x9csealsxe2x80x9d are held in place, and the spreading of said sealing material within the cavities is avoided.
The invention also relates to various devices.
It thus relates, firstly, to a radiation detector manufactured by means of one of the methods succinctly described above.
The invention relates, secondly, to a radiation detector comprising an assembly of microdetectors, for example microbolometers, said detector being remarkable in that it further comprises
a portion of substrate,
a peripheral partition surrounding said detector and joined to said portion of substrate, and
a wall adapted to serve as a window for said radiation and joined to said peripheral partition,
said portion of substrate, said peripheral partition and said wall forming an envelope for said detector, within which are located one or more cavities containing said microdetectors under vacuum or under low pressure.
The invention relates, thirdly, to a radiation detector comprising an assembly of microdetectors, for example microbolometers, placed under vacuum or under low pressure in one or more cavities located within the envelope of said detector, said detector being remarkable in that said envelope comprises
a portion of substrate, and
a wall adapted to serve as a window for said radiation, and in that said envelope has one or more openings sealed using a suitable material.
It will be noted that the devices according to the invention may be entirely manufactured using microelectronics techniques, that is to say in environments which give rise to very low levels of polluting particles, and that, moreover, the finished device may be cleaned by conventional techniques in case of need, during or after the final operations of soldering of the connections, or of soldering and gluing on the final support. Due to this the manufacturing yield is high.
According to particular features, the radiation detector comprises, on the one hand, a peripheral partition, and, on the other hand, either a network of partitions, or a network of internal pillars, said window being joined to said peripheral partition and to said internal pillars or partitions.
By virtue of these provisions, it is not necessary to modify the production technique to adapt it to the dimensions of the detector, as was the case in the prior art using a cover. The mechanical effort due to atmospheric pressure is compensated for by the internal pillars and/or partitions, independently of the dimensions of the detector.
Furthermore, the window may be of small thickness, which naturally leads to a high level of optical transparency, favorable to optimal performance.
According to particular features, some selected internal surfaces of the cavities are covered with a getter.
More particularly it may in this manner be useful to improve the durability of the interior vacuum, which is necessary in order to maintain the optimal characteristics of the bolometers. Although the technique of producing and sealing the cavities according to the invention makes it possible to achieve a sufficient level of vacuum without the use of getters, the person skilled in the art may consider, in certain circumstances, given the specifications aimed at (in particular concerning the longevity of the microdetectors), that the surface/volume ratio of the cavities so obtained is too high.
Once the manufacturing has been completed according to these provisions, the assembly is subjected to automatic electro-optical test and sorting operations, then is cut up into individual chips.
It will be noted in this connection that it is possible, by virtue of the invention, to implement these operations in an automated manner, directly at wafer level, and before cutting up into individual detectors; this is very advantageous in relation to conventional methods, in which these test operations are performed on individual detectors integrated into casings. All the unitary operations according to the prior art described above are thereby eliminated. The invention thus enables considerable savings to be achieved.
The detector can be used xe2x80x98as isxe2x80x99, like any microelectronics chip at this stage of elaboration. It is thus virtually possible to apply all the methods of integration to more complex systems in use in industry, provided that the mechanical and chemical aggressions during the process and the implementation are compatible with the relative superficial mechanical vulnerability of the detector in relation to the optical surface.
Said individual chips may be treated in various manners.
According to a first use of the chips according to the invention, they are integrated directly into their final site of implementation (xe2x80x9cbare chipsxe2x80x9d).
It is nevertheless necessary to take care that the component is treated in a manner compatible with its relative fragility, which depends on the nature and thickness of the window as well as on the spacing between successive internal pillars or partitions; more particularly, the less the spacing, the less the chip will be vulnerable as regards the window, for a constant thickness of this window.
According to a different use of the detectors according to the invention, they are integrated by soldering or welding in a casing provided with a window that is transparent to the radiation that it is sought to detect (and possibly provided with Peltier modules for the thermal stabilization). This casing is next used like any casing of a conventional device adapted for the detection of radiation.
Although the use of individual casings implies an additional cost (in exchange for an increased level of protection for the chips), it will be noted that the casings have no need for particularly strict specifications for air-tightness, as was the case in the state of the art. The usual specifications of casings for visible imaging, already widely distributed and thus relatively cheap, are adequate. It is sufficient to specify a window that is transparent to the wavelengths to be detected.
Finally the invention relates to diverse measurement or observation apparatus incorporating at least one radiation detector such as those succinctly described above. These apparatus may for example be imaging systems operating in the infrared range.
Other aspects and advantages of the invention will emerge from a reading of the following detailed description of particular embodiments, given by way of non-limiting example. This description refers to the accompanying drawings, in which: